Instrument centers for dentists, hereinafter called "dental units," incur significant microbe build-up at the water removal points for drills, turbines, syringes, and so forth. The source for this microbe build-up is commonly the tap water itself, or an ion exchange upstream in the system may be a breeding site for various types of microbes (bacteria, fungi, spores). The microbe source is most frequently, however, in the conduit system for the water, which include T-shaped and elbow sections. These conduits, with the customary and increasing use of plastic, provide excellent microbe growth media, particularly inasmuch as the component parts of the plastic conduit system can provide a nutritive base for microbes. Also, upon completion of dental treatment, that is when hand held tools (for instance, drills) are set down by the dentist or dental assistant, pathological microbes originating from patients can be easily drawn by a very slight vacuum into the conduit system. These microbes may include hepatitis A or B viruses, herpes viruses, HIV and so forth. Disinfection or sterilization filters, inversion osmosis devices and so forth represent other critical points for microbial growth.
Microbes which can become established and multiply in the water passages of dental units are predominantly pseudomonas aeruginosa, pseudomonades of the fluorescence group and legionellas; furthermore alcaligenes faecalis, flavonic bacteria and escherichia coli have also been detected (see BECK and SCHMIDT 1986, J. BORNEFF 1988, BOTZENHART and HEROLD 1988).
Lowering the microbe count to legally permissible levels with fewer than 100 colony-forming units/ml (KBE/ml) is possible, according to present practice, only by continuous disinfection and sterilization of the water. A one-time sterilization of the instrument by a chemical disinfecting agent or water vapor does not have a permanent disinfecting or sterilization effect, as microbes are always resupplied from the water or from the patient.